feat(vector-io): implement chunk deletion for PGVector provider

Implement delete_chunk() method with SQL DELETE operation
Add _delete_openai_chunk_from_vector_store() for OpenAI compatibility
Fix chunk_id generation to use actual chunk.chunk_id instead of generated IDs (so they can be removed by id)
Set pgvector provider to requiring Api.files as optional
Update remote_provider_spec() function to support optional_api_dependencies parameter

This allows pgvector to work in configurations without a files provider
while still supporting file operations when files API is available.

Signed-off-by: Derek Higgins <derekh@redhat.com>

llama_stack/providers/registry/vector_io.py

@@ -410,6 +410,7 @@ See [PGVector's documentation](https://github.com/pgvector/pgvector) for more de
 """,
             ),
             api_dependencies=[Api.inference],
+            optional_api_dependencies=[Api.files],
         ),
         remote_provider_spec(
             Api.vector_io,